I've lifted this from a UK board that I know some of you visit. Some worrying news, not only about the consequences of escaped farm fish, (which is to some extent already known, though this gives some hard figures) but also of deliberately stocked fish. There are implications not only for atlantic salmon, but also other species.

"15th October 2003
Accidental and Deliberate Introductions of Farm Salmon Result in Reduced Survival and Fitness and could Lead to Extinction of Vulnerable Wild Populations of Atlantic Salmon

Currently production of Atlantic salmon in the north Atlantic is some 700,000 tonnes per year. The physical nature of marine net cages is such that they are vulnerable to damage by storms, predators and collisions, and escapes from confinement inevitably occur. The largest single incident to date occurred in the Faroes in spring 2002 when 600,000 fish escaped during a storm accident. Fish are also lost during routine handling operations. It is estimated that some two million Atlantic salmon escape each year from fish farms in the North Atlantic, which is equivalent to about 50% of the total number of wild adult salmon in the sea. Escaped salmon can enter rivers where they breed and interbreed with wild salmon, thereby potentially changing the genetic make-up of wild populations. The importance of such changes in the survival of the remaining wild populations of Atlantic salmon has been a matter of debate for the past decade but little empirical evidence has been available.

In a paper published today in the prestigious scientific journal Royal Society Proceedings B, we report on the results from large-scale experimental simulations of farm salmon escapes*. The study, which was carried out at the Marine Institute facilities on the Burrishoole River system, Co Mayo, was directed jointly by Dr Philip McGinnity of the Marine Institute and Prof Andy Ferguson of the School of Biology & Biochemistry, Queen’s University Belfast. These experiments, which have taken 10 years to complete, examined, for the first time, multiple families of both first and second generation hybrids between wild and farm salmon in the freshwater and marine life history phases. Known numbers of eyed eggs of each group were planted together in a 7,250m2 section of a natural salmon-spawning river. Juveniles (parr and smolts) were subsequently sampled from this river and as emigrants at a downstream trap. Aliquots of eyed eggs from the same families were also reared in a communal hatchery tank and released to the open sea as smolts. Returning adults were captured in the coastal nets, by angling, and at the upstream traps. DNA profiling was used to identify the parentage of individuals. By carrying out the experiments under common-environment conditions, the effects of environmental variation are eliminated and thus any differences found are the result of differing genetic make-up.

The experiment demonstrated that farm Atlantic salmon have both genetic and competitive impacts on wild populations. As a result of domestication over several generations, genetic changes have reduced the capability of farm salmon to survive in the wild, especially during the marine phase. Overall farm salmon showed an estimated lifetime success of 2% of that of the native wild salmon, and results were consistent in three separate cohorts. Hybrids were intermediate in lifetime success between the wild and the farm (27% - 89% relative to wild). In the second generation hybrids, some 70% of the embryos died in the first few weeks as a result of outbreeding depression. This condition is caused by genetic incompatibilities between the parents but does not occur until the second generation when recombination of the parental genes has taken place. Taking our results alongside those available in the literature, it can be concluded that genetic changes leading to reduced survival in the wild is a feature of all domesticated salmon and trout and consequently hybrids between farm and wild fish also have reduced survival.

When mature farm salmon spawn in a river, although some pure farm offspring may be produced, most mating results in interbreeding between wild and farm salmon. Thus part of the potential wild juvenile production is converted to hybrids, which show reduced survival and adult return.

It was found that hybrids returned mainly after two winters at sea compared with one winter for the native salmon of the Burrishoole river. While the increase in these larger two sea winter hybrids could be an advantage from an angling point of view, the overall poorer survival of hybrids means it occurs at a cost to juvenile production in the next generation.
Farm salmon have been selectively bred for faster growth. In this study, juvenile farm salmon and hybrids grew faster than wild juveniles and competitively displaced the wild fish from the river. In one cohort, 57% of the wild parr were displaced. The poorer survival of farm and hybrid salmon at sea means that they do not compensate for these displaced wild juveniles and so there is an overall reduction in returning adults.

The combined impact of hybridization and competition means that, when a large number of farm salmon spawn in a river, the number of adult salmon returning to the river and the potential offspring production in the next generation are reduced. The degree of the impact will depend on various factors including relative numbers of wild and farm salmon, and juvenile habitat availability. As repeated escapes are now a common occurrence in some areas, a cumulative effect is produced generation upon generation, which could lead to extinction of endangered wild populations as a result of this ‘extinction vortex’.

In addition to accidental escapes, deliberate stocking of surplus Atlantic salmon from the farm industry is undertaken, especially in Scotland, in an attempt to supplement wild populations, which have become depleted as a result of environmental deterioration, habitat loss and over-exploitation. Deliberate introduction of farm salmon is likely to be more damaging than accidental escapes and have the reverse effect to that intended, with the wild population being further reduced. Thus when adult farm salmon enter a river it is largely hybrids that are produced, whereas in the case of deliberate stocking large numbers of pure farm juveniles are introduced in the first generation. These farm fish have a greater competitive effect and lower overall survival than hybrids. Also stocking may result in large numbers of farm fish being introduced annually. Farm stocks of brown trout, while not subject to the same intense programme of artificial selection for increased growth as Atlantic salmon, have been domesticated for a much longer period of time, over 100 years in the case of some strains. Stocking with farm brown trout is carried out on a much wider scale. Our results demonstrate that this brown trout stocking, as with Atlantic salmon, is detrimental to the fitness and survival of wild populations. In light of this study, and other similar studies with North American salmonids, the policy of deliberate stocking with domesticated salmonids needs urgent review.

Further information may be obtained from Philip McGinnity (pmcginnity@anu.ie; phil.mcginnity@marine.ie) or Andy Ferguson (a.ferguson@qub.ac.uk).
*Proceedings Royal Society London B (2003). DOI 10.1098/rspb.2003.2520. "